An infrared focal plane array (FPA) is an imaging sensing apparatus that includes an array of pixels capable of detecting photons in the infrared spectrum. The pixels of infrared FPAs are formed of a material that is sensitive to infrared radiation, such as indium antimonide (InSb), Mercury Cadium Telluride (MCT), or other infrared-sensitive detector materials. A common approach for fabricating two-dimensional FPAs is to connect the detector pixels to a read-out integrated circuit (ROIC) via pixel interconnects, which are typically indium bumps located between the detector pixel contacts and the inputs to the ROIC. The ROIC may include CMOS integrated circuitry such as a multiplexer or other similar circuit, for example. An FPA consisting of a detector array and ROIC may also be attached to another substrate material (the FPA substrate), which provides electrical connections from the external system electronics to the FPA. In order to function properly, cryogenic infrared FPAs operate at very cold temperatures (typically less than 100 Kelvin). Special design considerations are required for cryogenic FPAs to address thermally induced stresses in the devices that result from the temperature cycling process.
Two common types of infrared FPAs include backside-illuminated and frontside-illuminated designs. Backside-illuminated FPAs consist of a monolithic infrared detector material connected to a ROIC by means of electrical interconnects, such as indium bumps. The interconnect area is usually backfilled with epoxy to hold the structure together during subsequent fabrication steps such as thinning of the detector material. Backside-illuminated FPAs are usually limited in application to small to medium size FPAs (i.e., less than 1×106 pixels) because high stresses develop between the monolithic infrared detector material, the indium bumps, the ROIC and the epoxy during cryogenic thermal cycling of the FPA.
Frontside-illuminated, backside-reticulated FPAs, on the other hand, consist of an infrared detector material (such as InSb) that has been etched to physically separate the pixels in the detector array (i.e., reticulated). The detector array is attached to a transparent silicon support wafer to provide support for the array. The support wafer is transparent in the infrared spectral region. Like backside-illuminated FPAs, the detector array is connected to a silicon ROIC by means of indium bump pixel interconnects. However, unlike the backside-illuminated FPA design, epoxy backfilling is not required because the detector array and the ROIC are both primarily composed of silicon, and therefore the thermal properties of the entire FPA are matched, which results in minimal stress during cryogenic thermal cycling. Frontside-illuminated, backside-reticulated FPAs are robust, and can be thermally cycled thousands of times. This FPA design is scalable from small formats to very large formats. For example, infrared FPAs with greater than 4×106 pixels are in production.
Because pixels such as InSb and MCT, for example, detect infrared radiation effectively at cryogenic temperatures, the infrared FPA must capable of withstanding cryogenic thermal cycling, which is commonly between 373 Kelvin (100° C.) and 77 Kelvin. As described above, the pixels of frontside-illuminated, backside reticulated FPAs have a physical gap between pixels. Thermal cycling of the FPA has been shown to cause slow migration of the indium bumps into the gap between pixels. After many thermal cycles, indium from one pixel interconnect can migrate across the gap, and physically contact the adjacent pixel interconnect. This contact causes an electrical short between two pixels, forming a “pixel pair” defect. Eventually, if enough pixel-pair defects are formed, the FPA performance will be significantly degraded.
Accordingly, it is desired to provide improvements in FPA design and methods of fabrication that prevent the migration of indium and pixel pair formation due to cryogenic thermal cycling.